The present invention relates to lasers and non-linear frequency conversion techniques and, particularly, to a technique to convert infrared radiation to visible radiation using intracavity sum frequency generation.
Solid state lasers are a class of lasers which contain a solid state gain element. The gain element generally consists of a host material, which can be either a crystalline or amorphous or glass-like material, and a dopant or impurity ion distributed within the host material. The dopant ion, which is typically a transition element or rare earth element, is the primary determinant of the wavelength or wavelengths over which the laser can emit radiation. Typically, solid state lasers operate in the infrared region between 700 nm and 3 .mu..
However, it is desirable for numerous applications to use a visible laser. Because of the convenience of the solid state laser gain medium compared to either gaseous or liquid gain media, techniques have evolved to convert the infrared fundamental radiation to visible radiation. Non-linear optical conversion commonly is used to produce visible radiation from solid state lasers operating in the infrared (IR). Wavelengths in the blue are of particular interest for applications such as display technology, optical data storage, and underwater applications. The most common non-linear optical conversion technique is a process called second harmonic generation, or doubling. To achieve doubling the laser output is directed through a non-linear optical material. The laser beam emerging from the non-linear crystal is at one-half the wavelength of the initial laser beam. The initial laser wavelength is referred to as the "fundamental wavelength" and the doubled wavelength often is called the "second harmonic".
An important parameter for a non-linear crystal is the phase matching condition. Optimum conversion from the fundamental wavelength to the second harmonic wavelength will occur when the wave vector mismatch between the fundamental wave and the generated wave is zero. This condition is termed "phase matching". Phase matching may be achieved in an anisotropic crystal by a suitable choice of direction of propagation and polarization relative to the crystalline axes.
Two different types of second harmonic generation can occur in non-linear crystals. The second harmonic process can be thought of as mixing two waves of identical wavelength to produce a third wave at one-half the wavelength. In this context, Type I second harmonic generation refers to the process where the two fundamental waves have the same polarization. Type II second harmonic generation occurs when the fundamental waves have orthogonal polarizations.
Phase matching is achieved as a result of the dispersion of the non-linear crystalline host. Dispersion refers to the dependence of the refractive index of a given material on wavelength. Phase matching is achieved in second harmonic generation when the refractive index at the fundamental wavelength is equal to the refractive index at the second harmonic wavelength. Because of dispersion, the refractive indices at the two wavelengths can be equal if the material is birefringent. That is, the crystal must have a different refractive index for the ordinary and extraordinary waves. In this case a propagation direction may be chosen with respect to the crystallographic axes where for a given fundamental wavelength, the refractive indices for the second harmonic and fundamental wavelengths are identical.
The conditions for phase matching depend specifically on the desired non-linear operation. For example, second harmonic generation of the Nd:YAG laser wavelength from 1.064 .mu. to 532 nm requires that the refractive index of the non-linear crystal at 1.064 .mu. and the refractive index at 532 nm be identical. In such a case the phase relationship between the fundamental wavelength and the generated second harmonic wavelength remain unchanged as the two waves propagate along the length of the crystal.
When phase matched second harmonic generation is achieved by propagating the fundamental wavelength along a direction different from a principal axis of a birefringent crystal it is termed "critical phase matching." When critical phase matched second harmonic generation is used with a focused beam, there is a phase mismatch of the wave vector for small deviations from the phase match direction due to the finite divergence of the beam. However, since the efficiency of the non-linear conversion process is a function of the power density within the non-linear crystal, focusing is generally desirable to achieve high conversion efficiency.
When the phase matching angle is 90 degrees for a particular non-linear process in a given material, it is termed "noncritical phase matching" (NCPM). In such a case, effects of beam divergence vanish. That is to say, a strongly focused beam in an NCPM crystal does not have the phase mismatch problems as is evident in critical phase matching. In addition the walk-off angle, which is the direction of energy flow of the fundamental and second harmonic beams, is zero. This allows the two beams (the fundamental and second harmonic) to propagate collinearly within the crystal.
NCPM is therefore a desirable and potentially highly efficient type of phase matching. One means by which NCPM can be obtained is by adjusting the temperature of the non-linear crystal to the point where the refractive index of the fundamental wavelength equals that of the second harmonic wavelength for a particular angle of propagation with respect to the crystallographic axes. NCPM can also be achieved at room temperature for a given non-linear material and fundamental wavelength. Room temperature NCPM has the advantage of simplicity.
Second harmonic generation is a special case of a more general non-linear optical conversion process known as sum frequency generation (SFG). In second harmonic generation, two optical waves of the same wavelength are combined to produce a single wave of a wavelength one-half the original fundamental wavelength. In sum frequency generation two fundamental waves of different wavelengths are combined to produce a third wavelength. The wavelength produced by sum frequency generation is determined by the following equation: ##EQU1## where .lambda..sub.1 represents one of the fundamental wavelengths, .lambda..sub.2 represents the second fundamental wavelength, and .lambda..sub.3 represents the converted or summed wavelength. Second harmonic generation is a degenerate case of sum frequency generation, since .lambda..sub.1=.lambda.2. The fundamental principles of non-linear optics summarized briefly above are well known and are discussed in detail in the literature. See, for example, G. D. Boyd and D. A. Kleinman, Journal of Applied Physics, vol. 39, p. 3597, 1968.
Although doubling can be an efficient means for obtaining blue visible wavelengths, the non-linear optical material KTiOPO.sub.4 (KTP) is non-critically phase matched at room temperature for sum frequency generation at 808 nm and 1.064 .mu., see, for example, K. Kato, IEEE J. Quantum Electronics, vol. QE-24, p. 3, 1988. The generated wavelength is 459 nm. This blue wavelength is of particular interest for several applications because it is compatible with the Cs atomic resonance filter. The Cs filter has the properties of having an extremely narrow bandwidth (about 0.002 nm) and a very wide acceptance angle. Optical radiation only within the pass-band of 459 nm.+-.0.002 nm is transmitted through this filter, so that the solar background is largely eliminated. An optical detector used in conjunction with this filter will detect virtually no natural solar light.
As a consequence, the detector will be highly sensitive to 459 nm radiation from a transmitter even in the presence of full sunlight. Put another way, the noise rejection for such a detector is extremely high. The 459 nm wavelength is therefore desirable for optical transmission of a weak signal in the presence of a large solar background when used in conjunction with a Cs atomic resonance filter. This wavelength is also near the optimum transmission wavelength for underwater propagation.
There are several compelling advantages to recommend the SFG process in KTP over direct doubling of 918 nm to 459 nm. For one, the required fundamental wavelengths (808 nm and 1.064 .mu.) can be obtained efficiently, while 918 nm is difficult to generate efficiently. For SFG, Nd:YAG operates efficiently at 1.064 .mu. while several lasers, including Ti.sup.3+ :sapphire, AlGaAs laser diodes and Cr.sup.3+ -doped crystals produce efficient output at 808 nm. In addition, KTP is a robust, mature and efficient non-linear crystal which is readily available in excellent quality from a number of commercial suppliers. Crystals that are appropriate for second harmonic generation of 918 nm, such as KNbO.sub.3, are less robust or mature. And finally, KTP has an exceptionally wide angular and temperature bandwidth for room temperature (approximately 25.degree. C.) non-critical phase matched sum frequency generation; see, for example, the article by J. -C. Baumert, F. M. Schellenberg, W. Lenth, W. B. Risk and G. C. Bjorklund, Appl. Phys. Lett., vol. 51, p. 2192, 1987. Second harmonic generation from 918 nm is generally not NCPM at room temperature.
Typically, sum frequency generation requires two different laser sources. Since the efficiency of the sum frequency generation process depends on the power density (power per unit area) within the optical crystal, extremely small focused spot sizes within the non-linear sum frequency generating crystal typically are used. Using two different laser sources generally leads to problems involving the alignment of the beams to the high degree of accuracy required by these small spot sizes within the non-linear crystal. In addition, when using two separate laser sources, there are inefficiencies that result from mismatching of the spatial modes of the two lasers in terms of size, shape and intensity distribution.
One technique for avoiding the use of two separate lasers for the sum frequency generation process in which 808 nm and 1.064 .mu. are combined to produce 459 nm blue output is to use a laser diode pumped Nd:YAG laser which uses the residual (unabsorbed) 808 nm pump radiation from the laser diode for sum frequency generation, see for example, the article by W. P. Risk, J. -C. Baumert, G. C. Bjorklund, F. M. Schellenberg and W. Lenth, Appl. Phys. Lett., vol. 52, p. 85, 1988. In this type of sum frequency generation process a laser diode at 808 nm is used to pump a Nd:YAG laser which operates at 1.064 .mu.. The residual or unabsorbed 808 nm pump light is then circulated within the Nd:YAG laser resonator cavity which also includes a sum frequency generating KTP crystal. In such a system there is only one active laser, the laser diode. Since the Nd:YAG is optically excited by the laser diode and in essence serves as a frequency conversion device to convert some of the 808 nm light to 1.064 .mu. light, one might conclude that sum frequency generation is achieved with the use of only one active laser. A patent by Baumert et al., U.S. Pat. No. 4,791,631, describes this concept in detail.
A variation of the Baumert et al. type of sum frequency generation process is to have an additional laser diode or laser diodes which are not used to pump the Nd:YAG directly but are used to introduce additional 808 nm light into the laser resonator which contains the Nd:YAG crystal and the KTP nonlinear crystal. In this case a separate laser diode is used to pump the Nd:YAG laser.
It should be noted that for continuous wave (cw) sum frequency generation the use of the KTP crystal within a resonator is essentially a requirement which stems from the necessity of having very high power densities to achieve efficient generation of 459 nm light. Therefore, the KTP crystal used for cw sum frequency generation usually receives focused light at 808 nm and 1.064 .mu. within a laser resonator cavity. Using the KTP crystal inside a laser resonator cavity is desired because the circulating optical flux within a laser resonator cavity, which is forced by highly reflective end elements to oscillate back and forth, will have a much higher power than light outside the cavity.
A second patent by Dixon et al., U.S. Pat. No. 4,879,723, describes another version of the concept patented earlier by Baumert et al. In the Dixon et al. patent a laser diode pumped Nd:YAG laser is established in a laser resonator cavity containing the KTP crystal similar to the Baumert patent. In addition, the output of a second laser diode is introduced into this same cavity to provide a separate source of 808 nm power. In the Dixon et al. patent a possibility of high modulation rate of the 459 nm light is considered.
Sum frequency generation utilizing a laser diode has a unique set of difficulties, particularly for scaling to higher power. These difficulties stem from the broad spectral bandwidth and poor beam quality that typically is associated with high power laser diodes. Laser diodes with power output on the order of 1 Watt or more are typically multi-spectral devices. This broad spectral output limits the efficiency of sum frequency generation at a specific blue-green wavelength. In addition, these high power laser diodes typically are multi-transverse mode devices, as they arise from gain-guided wide-stripe architectures. As a consequence of the large number of transverse modes, it is not possible to focus the output of the high powered 808 nm laser diode into a small enough spot to produce efficient sum frequency generation. In addition, the spatial mismatch between the focus spot of a typically astigmatic high transverse mode laser diode and the TEM.sub.OO output of a Nd:YAG laser further prohibits good optical conversion efficiency for the sum frequency generation process.
For cw sum frequency generation in which 808 nm and 1.064 .mu. wavelength are summed to produce 459 nm, an additional problem arises in the types of approaches represented by the Dixon et al. and Baumert et al. patents cited above. In those approaches the laser resonator cavity in which the two fundamental wavelengths are resonated or circulated (in order to produce the high intracavity power desired for efficient sum frequency generation) also contains the Nd:YAG laser gain element. However, the Nd:YAG element absorbs strongly at 808 nm and therefore reduces the intracavity power at that wavelength. Subsequently the overall conversion efficiency is reduced.
Several variations on the general techniques proposed by the Dixon et al. and Baumert et al. patents have been published. All of these concepts suffer from the problem of having a Nd:YAG gain element which absorbs at 808 nm contained within the laser resonator cavity that is used to obtain high intracavity power at 808 nm. See, for example, the articles by D. W. Anthon and G. J. Dixon, M. G. Ressl, and T. J. Pier, Proceedings of the SPIE, vol. 898, p. 60, 1988; W. P. Risk and W. Lenth, Appl. Phys. Lett., vol. 54, p. 789, 1989; and P. N. Kean and G. J. Dixon, Optics Letters, vol. 17, p. 127, 1992.
Since good efficiency dictates the use of intracavity sum frequency generation (or "mixing") to take advantage of the high circulating power at the fundamental wavelengths, absorption of the 808 nm power by a Nd:YAG gain element contained within the laser resonator cavity will counteract the enhancement of the 808 nm power within the cavity and reduce the overall conversion efficiency. One solution to the problem had been suggested in which an optically thin Nd:YAG slab is used. Thin slabs have a shorter absorption length and therefore do not absorb as strongly in a single pass as longer Nd:YAG gain elements would. See, for example, the article by P. N. Kean and N. G. Dixon, Optics Letters, vol. 17, p. 127, 1992. The use of the optically thin slab mitigates the problem but does not completely eliminate the intracavity absorption of 808 nm power.
A different type of solution has been proposed in which sum frequency generation occurs in an external resonant cavity. In this type of approach the output of a laser diode-pumped Nd:YAG laser is introduced into a second resonator cavity which contains only a sum frequency generating KTP crystal. The 1.064 .mu. power from the diode-pumped Nd:YAG laser circulates in this cavity. In addition, the output of the laser diode at 808 nm is also introduced into the external cavity. Thus, both 1.064 .mu. and 808 nm fundamental radiation can circulate with high power density and produce efficient 459 nm radiation. See, for example, the article by W. P. Risk and W. J. Kozlovsky, Optics Letters, vol. 17, p. 707, 1992. However, it is to be noted that generation in an external resonant cavity, although circumventing the absorption problem in which the Nd:YAG gain element absorbs the 808 nm circulating power, introduces the alignment and mode matching difficulties which were referred to above. This technique for generating 459 nm light is therefore subject to losses in efficiency due to imperfect spatial overlap of the two beams from two separate lasers.
The above-referred to co-pending U.S. patent application Ser. No. 08/108,131 discloses a technique in which a single laser produces the two fundamental wavelengths simultaneously. The non-linear mixing crystal is contained within the laser resonator cavity to take advantage of the high circulating power. This technique for SFG not only circumvents the alignment problems associated with using two separate lasers, but also avoids the absorption problems associated with resonating 808 nm light in the presence of a strongly absorbing Nd:YAG gain element. However, in order to operate a laser at two wavelengths simultaneously, the net gain at the two wavelengths must be comparable. In general, the spectral dependence of the stimulated emission coefficient for a laser material (which determines the intrinsic gain of the material) makes it unlikely that the gain would be the same at both fundamental wavelengths for the SFG process generating 459 nm. Note that the two fundamental wavelengths, 808 nm and 1.064 .mu., are separated by over 200 nm.
The above-cited co-pending U.S. Patent Application discusses an exemplary Ti:sapphire laser. This laser material has a stimulated emission cross section at 1.064 .mu. equal to approximately 20% of the stimulated emission cross section at 808 nm. In order to achieve simultaneous operation at 808 nm and 1.064 .mu. in Ti:sapphire, the net gain at 1.064 .mu. must be enhanced relative to that at 808 nm.
Two enhancement techniques are described in the above-cited co-pending patent application. The first uses a low power optical signal at 1.064 .mu. which is injected into the Ti:sapphire laser resonator cavity. A second technique was described which uses a doubled Nd:YAG laser to pump the Ti:sapphire laser material. The pump beam is a dual wavelength beam which contains both 532 nm, the doubled output of Nd:YAG, and 1.064 .mu., the Nd:YAG fundamental wavelength. The 1.064 .mu. power is the un-doubled Nd:YAG power which would otherwise be filtered out of the pump beam. Both of these techniques have proven to be viable and effective means to accomplish the task of equalizing the net gain at the two fundamental wavelengths.
However, for the injection technique, alignment of the injecting source with the Ti:sapphire laser resonator cavity is critical. In addition, the injection source must be mode-matched to the spatial mode of the Ti:sapphire laser. The technique which uses a dual wavelength pump beam must also be mode-matched at 1.064 .mu. to the laser resonator cavity. Two lasers are said to be mode-matched over a certain spatial extent if the waist and divergence of two beams are similar.
More significantly, a cw, doubled Nd:YAG laser is not the laser source that is typically used for pumping the Ti:sapphire gain material. More commonly an argon ion laser is used, and it is desirable to develop a technique that can take advantage of the convenience of the argon ion laser pump source.
Thus in accordance with this inventive concept, a continuing need has been found in the state of the art for a technique for intracavity sum frequency generation using 808 nm and 1.064 .mu. to produce 459 nm output with a non-linear crystal composed of KTP which is efficient, scalable to high power, insensitive to alignment and mode matching considerations, arises from a single laser source which can be pumped by any suitable optical means, and contains no elements within the laser resonator cavity that reduce the intracavity power at 808 nm.